1. Field of the Invention
This invention relates to an ophthalmometer which can measure the curvature radius of the cornea of a subject's eye with high accuracy by simple operation.
2. Description of the Prior Art
The ophthalmometer is a device for measuring the size of an index image projected upon the cornea of an eye to thereby measure the curvature radius of the cornea. Generally, the cornea is an elliptical surface and has two curvature radii, namely, a maximum curvature radius and a minimum curvature radius orthogonal thereto. Measurement of these two curvature radii will hereinafter be referred to as "cornea astigmatism measurement." There have heretofore been many types of the cornea astigmatism measurement, each of which has its own merits and demerits. The most typical conventional methods of cornea astigmatism measurement will hereinafter be described by reference to FIGS. 1 and 2 of the accompanying drawings.
FIG. 1 shows what is called Sutcliffe type ophthalmometer, in which the image of an index 2 is formed on the cornea of a subject 1 by an illuminating system, not shown, and the image of the index reflected by the cornea enters a lens 3. An aperture plate 4 having four openings formed therein is disposed just behind the lens 3 so that a light beam is divided into four beams. Across two of these light beams, two declination prisms 5a and 5b are respectively disposed for movement in the direction of the optical axis and moreover, these prisms are oriented in such a manner that their bases are 90.degree. out of phase with each other and the base surfaces are parallel to the optical axis. When the image of the index 2 reflected by the cornea is focused on a focusing plate 6, three images of the index 2 may be seen through an eyepiece 7 because two of the four openings are aligned with each other. These three index images may be aligned with one another by moving the prisms 5a and 5b, and the amounts of movement of these prisms may be read as curvature radii, whereby cornea astigmatism may be measured.
Thus, the ophthalmometer of this type has an advantage that it can measure the curvature radii on the two axes of the cornea, but since the prisms 5a and 5b are moved between the aperture plate 4 and the focusing plate 6, the center thickness, deflection angles and directions of movement of these prisms 5a and 5b all affect the measurement, This makes the device not only difficult to manufacture but also poor in accuracy.
FIG. 2 shows another known device which is called Hartinger type ophthalmometer. A feature of the ophthalmometer of this type is that two sets of rotary prisms 5a and 5b each comprising a prism having a center portion performing a deviation opposite to that of the peripheral portion are disposed across a parallel light beam to separate into two images the index image reflected by the cornea. To form the parallel light beam, a first lens 3a is disposed in such a manner that the focal point thereof lies at the location whereat the virtual image of the index projected upon the cornea is focused. The images separated by the rotary prisms are passed through a second lens 3b and thereby focused on a focal plane 6. Thereupon, the rotary prisms 5a and 5b are rotated in the opposite directions by rotating means, so that the two separate images are aligned with each other in a predetermined relationship. A certain relationship is maintained between the angles of rotation and the curvature radius of the cornea and the angles of rotation are read, whereby the curvature radius may be known. According to such design, the above-noted disadvantages may be overcome since the prisms are merely rotated, whereas in the cornea astigmatism measurement of the subject's eye 1, it is only the curvature radius on one of the two axes that can be measured and therefore, measurement of the curvature radius on the other axis presupposes taking the trouble to rotate the entire optical system by 90.degree., which in turn means much procedure involved in the measurement.